Channel response calculation in an OFDM receiver

ABSTRACT

An efficient algorithm is described for use with OFDM receivers that characterizes the impulse response of a communication channel using frequency domain techniques that reduce computational and memory requirements, relative to time-domain cross-correlation methods, without sacrificing algorithm performance. An FFT engine is used to transform a time domain input sequence, the transformed sequence is multiplied by the conjugate of the expected sequence, the product of several sequences is averaged, then the FFT engine transforms the average back to a time domain sequence, the magnitude of which is the impulse response of the channel.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/950,111, filed Jul. 16, 2007, the specification of which is hereinincorporated by reference.

BACKGROUND

1. Field of Invention

The invention relates to characterizing communication channels.

2. Prior Art

In an orthogonal frequency division multiplex (OFDM) receiver, areal-time complex signal is typically converted to the frequency domainusing a Fast Fourier Transform (FFT), recovering the orthogonalsub-carriers in the process. The recovered sub-carriers are thencorrected with channel equalization, and frequency and timecompensation. The sub carriers are demodulated to recover the datamodulated onto the sub-carriers by the transmitter.

Characterization of the communication channel is needed to estimate theimpairments to the transmitted signals passing through the channel.Channel response, also called channel transfer function, is the actualsignal distorting characteristics of the communication channel. Channelestimation is the process of determining the channel response. Impulseresponse is the time domain version of the channel response. Onecharacteristic of particular interest is the channel echo profile, theresult of multiple signal paths due to signal reflections caused by thechannel.

A known data sequence is transmitted through the channel that can beused by the receiver to compute the channel response.

It is common practice in communication systems to use time domainmethods to compute the channel transfer function, which is representedby the channel impulse response. Computation of channel impulse responseby conventional methods requires performing correlation between thereceived signal and the known sequence, which is costly to perform insoftware. Time domain cross-correlation techniques require computationsthat increase significantly with the channel length and sequence length,requiring computations proportional to the product of channel andsequence length. A large amount of data memory is required in thereceiver to hold the received sequence during processing.

SUMMARY OF THE INVENTION

Channel characterization according to the present invention employs twoFFT operations to process a known probe sequence that has beentransmitted through the channel to compute the channel impulse response.This frequency domain processing technique computes the impulse responseefficiently and reduces the hardware and soft are computational burdenrelative to time domain techniques. The process makes use of existingFFT computational resources in an OFDM receiver for an additionalpurpose to extract channel information in the frequency domain andfurther processes the channel information to obtain the channel impulseresponse.

The time domain signal received from a known channel probe message ordata sequence is transformed to the frequency domain using an FFToperation. Then, the frequency domain complex conjugate of the knownprobe sequence is multiplied by the frequency domain version of thereceived signal. This produces the frequency domain response of thechannel. The channel response calculated from several probe sequencescan be averaged to reduce noise in the measurement. After computing thefrequency domain response, this data is transformed to the time domainusing another FFT operation. The magnitude of the FFT output produces areal-valued impulse response of the communication channel that passedthe probe messages from the transmitter to the receiver. The result canbe used to optimize the length of the cyclic prefix (CP) appended toOFDM symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram of a system for processing receivedinformation to determine channel impulse response.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a block diagram of a system for processing received signalsto determine the impulse response of the channel that the receivedsignals passed through between the transmitter and receiver. Anorthogonal frequency division multiplex (OFDM) receiver comprises signalprocessing blocks to process the time domain received signal intoreceived data. A fast Fourier Transform (FFT) calculation engine 120converts a signal between time domain and frequency domain. The FFTprocess can transform data between the time and frequency domains inboth directions.

Equalization 122 performs frequency domain equalization in order tocompensate for frequency domain distortions of the transmitted signal.At the start of every data packet, a known sequence of symbols istransmitted for the purposes of coarse estimating the channel responseand the receiver uses the errors measured in the received sequence toadjust equalizer coefficients to compensate for errors in the subsequentdata symbols. The equalization process is performed in two sequentialsteps: (1) coarse channel estimation and (2) channel equalization.

Channel estimation is the process of isolating and estimating thefrequency domain errors introduced into the transmitted signal by thechannel. Channel estimation is performed at the start of each packet byusing information in a message preamble composed of a known sequence ofsymbols to derive a complex set of equalizer coefficients that providesan estimate of the frequency domain distortions of the transmittedsignal.

Channel equalization is the process of applying the channel estimationcoefficients determined at the start of a packet to subsequent OFDMsymbols in order to compensate for frequency domain distortionsintroduced by the channel medium. After a channel estimate has beenobtained for a given packet, channel equalization is performed bymultiplying the received signal by the channel estimation coefficientsto produce an equalized signal.

Frequency and time correction 124 corrects carrier offsets and symboltiming errors in the QAM profile of the received signal. Well-knowntechniques including a decision-directed frequency and time trackingloop can be used. A data slicer 126 converts the multi-level quadratureamplitude modulated (QAM) signals on each carrier into data bits. Thedata bits extracted from each carrier are concatenated to form thereceived data stream or data block.

In the present invention, data selector 110 selects between the receivedtime domain signal 102 and a computed channel frequency responsefeedback signal 182. A control signal 104 selects the feedback signal toperform the impulse response calculation from the channel frequencyresponse.

The present invention provides a precise characterization of channelresponse that can be used to prevent the introduction of symbol errorsby the channel. The precise channel characterization is used in additionto the equalization 122 process described above. The calculation ofchannel response is performed as follows. The transmitter periodicallysends a real-time signal with a known data sequence. The receiverrecognizes the transmission of the known data sequence, which can be inthe form of a dedicated channel probe message or data patterns embeddedin other messages. The receiver compares the received signal with theknown transmitted signal in order to characterize the channel response.

To characterize the response of the channel, the receiver first convertsthe received signal 102 into frequency domain using the FFT engine 120of the OFDM receiver. When processing the input signal to compute thechannel response, the output of the FFT 120 is processed directly,bypassing channel equalization 122 and frequency correction and timecompensation 124 used for receiving data. The FFT output is representedas a series of complex numbers, each of which is comprised of an I and Qcomponent.

The FFT 120 output is multiplied by the complex conjugate of the knowntransmitted sequence 145, called EVM sequence because it is used tocompute the Error Vector Magnitude (EVM) as describe below, which isstored or generated in the receiver. This multiplication 140 yields thechannel frequency response.

The channel response calculated from several identical probe sequencescan be averaged in order to reduce noise in the measurement of thechannel response. These sequences are averaged in the EVM Average Memory160. Switch 142 governs which data symbols within each OFDM symbol of areceived packet are added into the average of channel response undercontrol of the signal ignore_bl_spctrl.

The echo profile probe used to determine the impulse response of thechannel can be a pseudo-random sequence that is generated at the probesender and an identical sequence is generated in the receiver forcomparison with the sent probe. The sequence is the payload of a probemessage and can be transmitted as a single-carrier, time-domain signal.The sequence can be generated by a well-known linear feedback shiftregister with generator polynomial X¹⁰+X⁷+1. The shift register of thesequence generator can be initialized at the start of each probemessage, for example with 0×023, to create a predictable sequence. Thereceiver shift register is similarly initialized. The resulting startingsequence output from the PN generator is {1,1,0,0,0,1,0,0,0 . . . }. Thelength of the sequence can comprise the first N values generated, forexample, 1280 values. The sequence can be transmitted with pi/4-offsetBPSK modulation using single carrier modulation at a sample rate of 50MSPS.

Error Vector Magnitude (EVM) is a vector with both I and Q componentsthat represent the distance and angle between the constellation pointtransmitted and the constellation point received in the QAM (I,Q) plane.EVM is computed using special probe packets on a per-carrier basis asthe difference between the known transmitted sequence and the receivedsequence. EVM is used to determine the bitloading for each packet, thatis, the order of magnitude modulation (16QAM, 64QAM, etc) that can beused reliably on each subcarrier. The data sequence used for thebitloading EVM calculation can be the same sequence used for channelcharacterization.

Data is processed and stored in the EVM AVG Memory 160 using sufficientrange and resolution to avoid clipping signals or degrading thesignal-to-noise ratio. For example, a 20-bit wide path carries a 10-bitrepresentation for 1 and Q data. Number representation can be signedtwo's-complement or other format.

In a static environment, where the channel response is not varyingsignificantly over time, the results from subsequent FFT blocks can beaggregated, or averaged, to reduce system noise. This produces theaverage channel frequency response. For example, a memory array 160 of256 locations can be used to store the running sum of each sub carriercalculation. Switch 162 disables the feedback from the memory outputduring the first symbol to be summed to effectively reset the average.If the switch 162 is set to enable the feedback path for the firstsymbol of subsequent packets, the averaging occurs across severaldifferent packets.

Operation 150 {I,Q} signifies that the I and Q components of the complexproduct of the conjugate EVM sequence and the frequency transformedreceived sequence for each subcarrier are kept separate when they arestored and averaged separately in memory 160.

After the desired number of symbols is averaged, the average channelfrequency response result stored in EVM Average Memory 160 is passed onfeedback path 182 through the FFT via data selector 110 which iscontrolled by control signal 104. This final FFT processing stepconverts the channel frequency response to an impulse response. Block170 computes the magnitude of the impulse response (I²+Q²) from thecomplex result of the final FFT pass. The impulse response is a sequenceor array of values.

The impulse response can then be processed by an embedded processor orhost processor to extract information about the transmission channelsuch as time delay, delay spread, relative attenuation, and othercharacteristics. Subsequent processing is simplified using the inventionbecause autocorrelation does not need to be performed by software. Thehardware performs the equivalent of autocorrelation when the conjugateEVM sequence is multiplied by the received sequence and the accumulateddata representing channel frequency response is passed through the FFTto produce the channel impulse response. Path characteristics such asrelative attenuation of signal path and time delay can also be derived.The path characteristics are used to determine packet parameters such asdelay, echo compensation factors, and other characteristics that can beused to prevent errors from being introduced into packets bytransmitter, channel, or receiver characteristics. One significant echocompensation factor is cyclic prefix, which is a repetition of a certainportion from the end of the OFDM symbol that is placed at the start ofthe OFDM symbol. The prefix time, or length, is a function of themultipath component of the signal determined by the channel estimationaccording to the present invention

The present invention produces a high-precision estimate of the channelwhich can be used to prevent, reduce, or eliminate transmission signaland data errors.

Using this frequency-domain technique has the following advantages:

-   -   1. It reduces the computational burden from O(n²) to O(n*log(n))        where the function O(x) is defined as the order of x. In other        words, the frequency-domain technique reduces the number of        computations from a function of n² to a function of n*log(n).    -   2. It re-uses the existing FFT hardware        -   a. computations are performed in hardware at data rates            which are orders of magnitude faster than can be achieved in            software        -   b. memory required to estimate channel impulse response is            reduced by more than a factor of 10 compared to a software            implementation of a similar algorithm        -   c. functionality (logic) added to reuse existing OFDM            receiver hardware is negligible

The mathematical proof of the method employed by the invention is asfollows:

Receive Signal 102

s _(rx)(t)=s _(tx)(t){circle around (×)}h(t)

where h(t) is the channel characteristic.

After FFT:

s_(rx)(t)

S_(rx)(f)

-   -   where

S _(rx)(f)=S _(tx)(f)H(f)

-   -   After conjugating with the known transmit sequence,

S _(rx)(f) S _(tx)(f)=S _(tx)(f)H(f) S _(tx)(f)

S _(rx)(f) S _(tx)(f)=H(f)

Taking the second FFT:

H(f)

h(t)

1. A method of characterizing a communication channel comprising the steps of: receiving a known time domain signal transmitted through a channel; transforming the received time domain signal into a frequency domain signal using an FFT computation; multiplying the frequency domain signal by the complex conjugate of the known signal to produce a frequency domain channel response; passing the frequency domain channel response through the FFT computation to produce an impulse response of the channel; and computing the magnitude of the impulse response.
 2. The method of claim 1 further comprising: averaging the frequency domain response of the channel over a plurality of symbols before passing the frequency domain channel response through the FFT computation.
 3. The method of claim 1 where the FFT computation is part of an OFDM receiver signal processor.
 4. A device for computing the impulse response of a communication channel comprising: an FFT engine for computing the FFT of an input sequence to produce an output sequence; a data selector feeding the FFT engine to select between an input signal and a feedback path; a multiplier that multiplies the FFT output sequence by a predetermined EVM sequence: an averaging memory to average the multiplier output, the averaging memory feeding the feedback path; a magnitude calculator for computing the magnitude of complex numbers coupled to the FFT engine output; wherein when the data selector selects the input signal, the averaging memory averages the product of the EVM sequence and the FFT of the input signal, and when the data selector selects the feedback path, the FFT engine computes the FFT of the averaged multiplier output and the magnitude calculator computes the magnitude of the FFT output to produce the impulse response of the communication channel. 